In a continuously functioning, mechanical fuel injection system, so-called starting enrichment by means of a cold-start valve is known, which injects an additional fuel quantity into the intake manifold during the starting process in accordance with the position of a thermo-time switch. For example, the technical instruction booklet entitled "Fuel Injection Continuous Injection System (CIS)," published in 1974 by Robert Bosch GmbH, D-7 Stuttgart 1, Postfach 50, West Germany, on page 16 therein, describes an auxillary starting assembly which consists essentially of an electromagnetically-operated start valve for spraying additional fuel into the common intake manifold of an internal combustion engine, which is switched on at the beginning of the engine starting process, and a thermal-time switch, which is mounted on engine block to sense engine temperature and which limits the duration of time that the start valve is opened, or, at higher engine temperature, prevents this valve from opening at all. The thermal-time switch closes or opens the power circuit leading to the start valve depending on the engine temperature. Diuring cold starting, the power circuit is interrupted depending on the temperature of a bimetallic strip which is heated by a heating coil during the starting operation. When the switching temperature is reached, the thermal-time switch is opened, cutting off power to the start valve, and the heating coil maintains the switch open until the end of the starting operation. When rapid heating of the bimetallic strip is desired, the thermal-time switch may include a second heating coil which is switched off when the thermal-time switch opens.
This known special metering apparatus, with its one-time on-off switching, canot be adapted precisely enough to a required additional fuel quantity. Also, it has proved to be desirable for the special fuel metering apparatus to function not only in the event of starting a cold internal combustion engine, but in other events as well.